Neural stem cells are self-renewing multipotent progenitors that give rise to neurons, astrocytes and oligodendrocytes in the central nervous system (CNS). However, to date it remains unclear whether there exists a generic neural stem cell, as found in the hematopoietic system. It appears that the CNS consists of heterogenic stem cells that, although restricted in their potency, all retain the ability to self-renew, differentiate, and express of a set of universal markers. To understand exactly what characteristics define a neural stem cell it is first necessary to elucidate the lineage relationship between the various types of stem cells and how they contribute to the formation and maintenance of the central nervous system. In this application we propose genetic and cellular assays to determine the contribution of SOX2-expressing cells and function of SOX2 during adult neurogenesis. The experiments are based on our findings that SOX2 universally marks cells with in vitro stem cell potential isolated from all stages of mouse CNS ontogeny (Ellis, 2004;Brazel, 2005). We will take advantage of this neural stem/progenitor specific SOX2 expression to investigate the cellular identity of adult multipotent neural stem cells (NSCs) directly in vivo using genetic lineage tracing and cell-ablation strategies in the mouse. Furthermore, we have recently demonstrated that SOX2 functions to maintain embryonic and retinal neural progenitor identity (Graham, 2003;Taranova, 2005). We therefore hypothesize that the molecular signaling pathway regulated by SOX2 to define neural stem/progenitor identity during embryogenesis also acts to maintain their cellular and molecular profile throughout ontogeny. To address this we propose a conditional mutagenesis approach to test the role of SOX2 specifically in adult neural progenitors. At the end of the funding period we will have a comprehensive view of fate and role of SOX2 expressing cells and the function of the SOX2 signaling cascade in regulating neural stem cell identity and differentiation in the adult CNS. Understanding the origin, fate and function of NSCs will advance efforts to manipulate NSCs for therapeutic purposes.